A hierarchized storage apparatus, which comprises multiple types of physical storage devices with different performance and so forth, has been known for some time.
For example, in Patent Literature 1, there is disclosed a method (a method for carrying out a volume migration) for an administrator to distribute multiple different types of logical units (hereinafter called LU) to multiple tiers, and to migrate data from a LU of a certain tier to a LU of a different tier in accordance with a data I/O (Input/Output) frequency (the LU may be called a “logical volume” or simply a “volume”). According to a volume migration that conforms to the Patent Literature 1, data with a high I/O frequency is migrated from a first LU, which is storing this data, to a second LU (a LU that belongs to a tier of a higher level than the tier to which the first LU belongs) based on an expensive physical storage device that features high performance and reliability. Alternatively, data with a low I/O frequency is migrated from a third LU, which is storing this data, to a fourth LU (a LU that belongs to a tier at a lower level than the tier to which the third LU belongs) based on an inexpensive physical storage device that features low performance and reliability.
Furthermore, for example, in Patent Literature 2, there is disclosed a method for carrying out a page migration. The Patent Literature 2 discloses the following. That is, there is a virtual LU (hereinafter the TP-LU) that conforms to Thin Provisioning. There is a hierarchized pool that comprises multiple types of LU. Each LU comprising the pool is partitioned into multiple pages (storage areas), and as such, the pool comprises multiple pages. A page is allocated to a TP-LU write-destination virtual area, and write-target data is written to the page. The data in the pool is reallocated in accordance with the data I/O frequency. Specifically, data with a high I/O frequency is migrated from a first page, which is storing this data, to a second page, which is in a higher tier than the tier of the first page. Alternately, data with a low I/O frequency is migrated from a third page, which is storing this data, to a fourth page that is in a lower tier than the tier of the third page.